I'm not sure what my normal idle pressure has been but when it got my attention it was at 20".
Idle is at 650 rpms.
Very small throttle movements gets the gauge moving, I don't think it fluctuated so much in the past.
Here is what I know so far.
The gauge matches pressure altitude pretty close.
I used a vacuum pump to check the gauge and the tubing feeding it from the intake....holds tight.
Used a vacuum cleaner to put pressure to the intake through the carb, soap and watered the whole system, found some small leaks in the intake hoses...sealed them up....but not the problem.
Throttle shaft in the carb was making bubbles....I haven't looked to see when the carb was installed but it looks new (600 hrs on engine) no play in shaft to housing.
Did a compression test tonight, no intake valve leaks.
A local engine guy looked it over and thought 2 cylinders had lifters that were week and suggested Marvel in the oil....put in a quart and have one hour on it so far with no change.
Not sure where else to look....shouldn't I be below 15"?

How's your idle mixture setting, might be a little rich? Should be about 10 to 30 rpm rise at 1000 rpm when leaning slowly (book figures may be slightly different depending on engine/airframe/DALT combo, but this generally works fine) Some engines like a rich idle some lean so set it to what yours runs best at. As long as it accelerates nice without stumbling. Watch the MAP as you lean the engine at idle, should decrease as you lean and increase as you richen. Idle speed and mixture settings have a big effect on idle MAP

If you've checked everything else with no issues, induction leaks, cylinder leakdowns etc and it's running well and making power then I wouldn't worry too much about a 20" idle

If the gauge is reacting quickly to very small throttle movements then could be the damping in the gauge or line restrictor missing

delete

Try this Terry

http://www.advancedpilot.com/downloads/prep.pdf

Does it run good?

There may be oil/water in the lines? Happened a few times to me on twins and singles. It will mess up readings on MP gauge. If not corrected will require replacement of gauge after cleaning of tubing.

Last time it happened it was from preservative oil being distributed into the cylinder. New MP gauge was installed, tubing cleared with compressed air and reconnected. Works good. I learned not to spray preservative oil into #2 cylinder unless TDC!

Thanks for the help!
O-470L runs good, makes good power. Maybe a little rough at idle, but smooth above 1000rpm.
Mixture is close to 50 rpm rise at low idle, I will play with that.
I'll blow out the line from the gauge.
Thanks!

Ummm, not to be flippant, but why would you care?

I'd kinda like to see some MP at full lope, but idle.

MTV

mtv wrote:Ummm, not to be flippant, but why would you care?

MTV

Maybe I shouldn't care?
It does make full pressure at WOT.
I think it has changed at idle and don't want to ignore it.

mtv wrote:Ummm, not to be flippant, but why would you care?

I'd kinda like to see some MP at full lope, but idle.

MTV

I'd say that any change in the gauge's performance anywhere in the measurable range should be looked into. If it can't handle full vacuum, perhaps that's indicative that a more severe failure is impending that might affect accuracy at WOT.

It seems to me that this issue at worst is something to do with the instrument. At idle, why would I care what my manifold pressure is? Perhaps there's a leak in a line, or the gauge, but that's not going to cause the airplane to come out of the sky..... For indication of power, I've always used fuel flow...if your engine is flowing fuel, it's making power. At idle, the tach is the primary power instrument....

It would be interesting to find out what the cause is, in any case.

MTV

If you read the link I sent it explains, when not running you see air pressure say at sea level 29.92". When you start your motor and let it idle your throttle is not open there fore you have a restriction in between your carb and the things sucking air (cylinders) and you get a lower pressure at idle. When you push that throttle in and open the restriction your manifold pressure comes up but will average what is ambient and wheat is reduced by the sucking cylinders.

Glidergeek wrote:If you read the link I sent it explains, when not running you see air pressure say at sea level 29.92". When you start your motor and let it idle your throttle is not open there fore you have a restriction in between your carb and the things sucking air (cylinders) and you get a lower pressure at idle. When you push that throttle in and open the restriction your manifold pressure comes up but will average what is ambient and wheat is reduced by the sucking calendars.

I think OP knows this, which is why he's stating his apparent issue is that he sees an abnormally high MP reading at idle (20 in-hg) when he is expecting less than 15 in-hg. To me that means a leak and the gauge isn't reading accurately since some of that suck is being lost.

Check the line to mp gauge. Was working on an R22, having incorrect mp readings, turned out something rubbed through the metal line slightly, and a pin hole had developed.

Tom

I read Deakin's article on manifold pressure.
Here are a couple of quotes from it,

In most engines, idle MP will be around 12 inches or so, less than half the sea-level.

So if you're used to seeing 12 inches of MP at idle at your home airport and one day you see 15 inches instead, suspect an induction system leak.

If my airspeed indicator was off I would agree to not worry about it....it won't make the airplane fall out of the sky.
Because the MP gauge is an engine instrument and its indicating 20" at idle, I don't think I should ignore it.
If we were a bunch of 5000ft pavement pounders it would be less concerning.
Most of the advice I've gotten so far is to not worry to much about it, I'll probably put the cowl back on and keep an eye on it.
Don't sweat the small shit.

Tom wrote:Check the line to mp gauge. Was working on an R22, having incorrect mp readings, turned out something rubbed through the metal line slightly, and a pin hole had developed.

Tom

I pulled the line off the intake manifold and pulled vacuum to the gauge, it held tight.

That's got me wondering cause I've never paid attention to my idle MP

Glidergeek wrote:That's got me wondering cause I've never paid attention to my idle MP

I really haven't either. I'm not sure what it has always read.
But it was easily noticed with the needle damn near straight up and dancing around with very small throttle changes.
MP acts fine in flight, pull the power and it will drop to 10" or less.

You are doing the right thing. Manifold Instrumentation, in my book, needs to work.

mtv wrote:It seems to me that this issue at worst is something to do with the instrument. At idle, why would I care what my manifold pressure is? Perhaps there's a leak in a line, or the gauge, but that's not going to cause the airplane to come out of the sky..... For indication of power, I've always used fuel flow...if your engine is flowing fuel, it's making power. At idle, the tach is the primary power instrument....

It would be interesting to find out what the cause is, in any case.

MTV

Just me but I wouldn't use fuel flow as an indication it's making rated power... MAP and RPM are the ones to use. MTV I assume you have an injected TCM? You can have the unmetered and metered fuel pressures well out of spec but still make rated MAP and RPM, not cool on a turbocharged engine and to a slightly lesser extent a naturally aspirated engine if the pressures are too low and therefore too lean... Or you could have the pressures too high and get 200pph flow on your IO-520, powers gonna be down a bit!

I think it's worth checking out low MAP at idle... Wouldn't want to try a go around and have the engine stumble or quit as you power up and ditch it in the trees... I spend quite a bit of time setting up the big injected TCM engines to get them spot on and it's worth the effort when you can smack the throttle (within reason of course!) and they accelerate nice. Often they'll quit on finals or roll out if not setup right

But if it idles and accelerates well, makes rated MAP and RPM and the fuel flows are right then should be fine if there's no induction leak, holes in the MAP line or the gauge isn't rooted.

Not much to setup on a carb engine apart from idle mixture. Worth checking the induction couplings and crossover tube if you ever have a backfire, they can pop off and get a big leak..

Just my NZ2cents (US1.65cents) worth